Measuring apparatuses that are designed for continuously tracking a target point and coordinatively determining the position of this point can generally be combined under the term laser tracker, particularly in connection with industrial surveying. In this case, a target point may be represented by a retroreflective unit (e.g. cube prism) that is targeted using an optical measurement beam from the measuring apparatus, particularly a laser beam. The laser beam is reflected back to the measuring apparatus in a parallel manner, the reflected beam being sensed using a sensing unit of the apparatus. In this context, an emission direction and a reception direction for the beam are ascertained, for example by means of sensors for angle measurement that are associated with a deflection mirror or a targeting unit of the system. In addition, with the sensing of the beam, a distance from the measuring apparatus to the target point is ascertained, e.g. by means of propagation time measurement or phase difference measurement.
In addition, laser trackers according to the prior art may be embodied with an optical image capture unit with a two-dimensional, light-sensitive array, e.g. a CCD or CID camera or a camera based on a CMOS array, or with a pixel array sensor and with an image processing unit. In this case, the laser tracker and the camera may be mounted in particular one on top of the other such that their positions relative to one another cannot be altered. By way of example, the camera is arranged so as to be able to rotate together with the laser tracker about the essentially perpendicular axis of the latter, but so as to be able to be pivoted up and down independently of the laser tracker and hence so as to be separate from the optical system of the laser beam, in particular. In addition, the camera may—e.g. on the basis of the respective application—be embodied so as to be able to be pivoted about only one axis. In alternative embodiments, the camera may be installed in an integrated design together with the laser optical system in a common housing.
With the capture and evaluation of an image—by means of an image capture and image processing unit—of what is known as an auxiliary measuring instrument with markings whose relative location with respect to one another is known, it is possible to deduce an orientation for an object (e.g. a probe) arranged on the auxiliary measuring instrument in space. Together with the determined spatial position of the target point, it is furthermore possible to precisely determine the position and orientation of the object in space absolutely and/or relative to the laser tracker.
Such auxiliary measuring instruments can be embodied by what are known as contact sensing tools that are positioned with their contact point on a point of the target object. The contact sensing tool has markings, e.g. light points, and a reflector, which represents a target point on the contact sensing tool and can be targeted using the laser beam from the tracker, the positions of the markings and of the reflector relative to the contact point of the contact sensing tool being precisely known. In a manner known to a person skilled in the art, the auxiliary measuring instrument may also be a, for example handheld, scanner equipped for distance measurement for contactless surface surveying operations, the direction and position of the scanner measurement beam used for the distance measurement relative to the light points and reflectors that are arranged on the scanner being precisely known. Such a scanner is described in EP 0 553 266, for example.
For distance measurement, laser trackers in the prior art have at least one distance measuring device, said distance measuring device possibly being in the form of an interferometer, for example. Since such distance measuring units can measure only relative distance changes, what are known as absolute distance measuring devices are installed in today's laser trackers in addition to interferometers. By way of example, such a combination of measuring means for distance determination is known by means of the product AT901 from Leica Geosystems AG. The interferometers used for the distance measurement in this connection primarily—on account of the long coherence length and the measurement range permitted thereby—use helium-neon gas lasers (HeNe lasers) as light sources. A combination of an absolute distance measuring device and an interferometer for determining distance with an HeNe laser is known from WO 2007/079600 A1, for example.
Furthermore, in modern tracker systems—increasingly as standard—a sensor is used to ascertain a deviation in the received measurement beam from a zero position. This measurable deviation can be used to determine a position difference between the center of a retroreflector and the impingement point of the laser beam on the reflector and to correct or readjust the orientation of the laser beam on the basis of this discrepancy such that the deviation on the sensor is decreased, in particular is “zero”, and hence the beam is oriented in the direction of the reflector center. The readjustment of the laser beam orientation allows continuous target tracking (tracking) of the target point to take place and the distance and position of the target point to be continuously determined relative to the measuring appliance. In this case, the readjustment can be realized by means of a change of orientation for the deflection mirror, which can be moved in a motorized manner and is provided for the purpose of deflecting the laser beam, and/or by pivoting the targeting unit that has the beam-guiding laser optical system.
For continuous target tracking, laser trackers according to the prior art regularly have a tracking area sensor in the form of a position-sensitive detector (PSD), with measurement laser radiation reflected at the target being able to be detected thereon. In this connection, a PSD is intended to be understood to mean an area sensor that operates locally in the analog domain and that can be used to determine a focus for a light distribution on the sensor area. In this case, the output signal from the sensor is produced by means of one or more photosensitive areas and is dependent on the respective position of the light focus. Downstream or integrated electronics can be used to evaluate the output signal and to ascertain the focus. In this case, the position of the focus of the impinging light point can be ascertained very quickly and with a very high resolution. However, the PSD can be used to ascertain only a focus of the light distribution, and not a distribution of a plurality of light points.
This PSD can be used to determine a deviation in the impingement point of the sensed beam from a servo control zero point, and the deviation can be taken as a basis for readjusting the laser beam to the target. For this purpose and in order to achieve a high level of precision, the field of view of this PSD is chosen to be comparatively small, i.e. to correspond to the beam diameter of the measurement laser beam.
Sensing using the PSD takes place coaxially with respect to the measurement axis, as a result of which the sensing direction of the PSD corresponds to the measurement direction. The PSD-based tracking and the fine targeting can be applied only after the measurement laser has been oriented to a retroreflective target.
The target tracking described needs to be preceded by coupling of the laser beam to the reflector. To this end, a sensing unit having a position-sensitive sensor and having a relatively large field of view may additionally be arranged on the tracker. Furthermore, appliances of the type in question incorporate additional illumination means that are used to illuminate the target or the reflector, particularly at a defined wavelength that differs from the wavelength of the distance measuring means. In this connection, the sensor may be in a form that is sensitive to a range around this particular wavelength, for example in order to reduce or completely prevent extraneous light influences. The illumination means can be used to illuminate the target, and the camera can be used to capture an image of the target with an illuminated reflector. The mapping of the specific (wavelength-specific) reflex on the sensor allows the reflex position in the image to be resolved and hence an angle relative to the capture direction of the camera and a direction to the target or reflector to be determined. An embodiment of a laser tracker having such a target searching unit is known from WO 2010/148525 A1, for example. However, this embodiment has no functionality for ascertaining the spatial orientation of an auxiliary measuring instrument.
A disadvantage of laser trackers in the prior art is the need to use at least two separate optical components for sensing the orientation of an auxiliary measuring instrument and for target tracking. Not only does this increase the involvement in terms of materials and design, and hence the production costs, but it also makes the tracker larger and heavier and hence impairs ease of transport and handling for the user.